1. Field of the Invention
The present invention relates to a technique for converting a color signal in an object color space such as L*a*b* or RGB, and a black colorant quantity associated with the color signal, into the color signal in an output color space containing the black colorant such as YMCK, or a technique for converting the color signal in the input col or space containing the black colorant such as YMCK into the color signal in the output color space containing the black colorant such as YMCK.
2. Description of the Related Art
In performing the color printing of a color image by electrophotography, four colors of yellow (Y), magenta (M), cyan (C) and black colorant (K) are normally used. A typical color signal lies on the three dimensional color space such as L*a*b*, L′u′v′ color space independent of the device, or RGB color space for use with a monitor signal. Accordingly, it is required to convert a color signal on the three dimensional color space into the four dimensional color space in performing the color printing of the color image. However, this conversion occurs between different dimensions, not corresponding to one to one, in which the three-color color signal on the three dimensional color space and the four-color color signal on the four dimensional space that reproduces the three-color color signal are combined in plural ways.
There are two methods for deciding a combination between the three-color color signal on the three dimensional color space and the four-color color signal on the four dimensional color space, including calculating the black colorant quantity contained in the three-color color signal of Y, M and C, removing the under colors and adding the black colorant (K), and firstly deciding the black colorant (K) in some way, and deciding the quantity of Y, M and C in accordance with the black colorant (K) quantity.
For example, with a method as described in JP-A-Hei. 5-292306, first of all, the K quantity (achromatic black colbrant quantity) of the four-color color signal for reproducing an object color signal under the condition of Y=0%, M=0% or C=0% is weighted with a preset factor to decide a new K quantity. And the quantities of Y, M and C for reproducing the object color signal are decided in accordance with the K quantity. Thereby, the black colorant quantity can be controlled to realize the color reproduction at high precision while achieving the purpose.
Also, with a method as described in JP-A-Hei. 6-242523, first of all, the K quantity (maximum black colorant quantity) of the four-color color signal for reproducing an object color signal under the condition of Y=0%, M=0%, C=0% or K=100% is calculated. Also, the K quantity (minimum black colorant quantity) of the four-color color signal for reproducing the object color signal under the condition of Y=100%, M=100%, C=100% or K=0% is calculated. Employing the maximum and minimum black colorant quantities thus calculated, a new K quantity is decided with the preset parameters. And the new quantities of Y, M and C for reproducing the object color signal are decided in accordance with the K quantity. Thereby, it is possible to utilize the color gamut reproducible with four colors including the block colorant to the maximum.
Herein, a condition called a coverage restriction is imposed on the general output device. The coverage restriction is an upper limit for the total quantity of recording material such as the toner or black colorant for use in reproducing the color signal. Mainly, the coverage restriction is imposed to avoid a degradation in the reproduction performance or an upsurge on the print surface due to excessive use of the recording material such as the toner or black colorant, or protect the output device.
With the conventional method as above described, since no coverage restriction is considered, when the black colorant quantity is controlled with the preset parameters, the color signal may not be reproduced irrespective of reproducible color gamut. That is, if the color signal is recorded with the calculated values of Y, M, C and K, the color gamut is consequently compressed due to a degradation in the reproduction performance, resulting in a problem that the color reproducibility is degraded.
In order to solve the above-mentioned problem, in JP-A-2002-010096 (US2001/0035968A1), the optimum black colorant quantity is calculated in the color gamut reproducible of three colors, the outermost contour of four color gamut satisfying the coverage restriction is searched from outside the color gamut reproducible of three colors, and the black colorant quantity K is decided using the black colorant quantity on the outermost contour and the optimum black colorant quantity calculated previously. Employing the black colorant quantity K calculated in this manner, a combination of Y, M, C and K satisfying the coverage restriction can be obtained.
Generally, the optimum black colorant quantity is designed smaller when the chroma component of the object color signal increases. Because a natural color reproduction is made by reducing the black colorant quantity with the increasing chroma component. However, with the method as described in JP-A-2002-010096 (US2001/0035968A1), the maximum black colorant quantity is obtained as the black colorant quantity in the outermost contour of color gamut satisfying the coverage restriction, although the black colorant quantity within the three color gamut is adjusted in the above manner. Therefore, in the achromatic color, the coverage restriction is satisfied by the calculated Y, M, C and K, and the color gamut is fully utilized, but the black colorant quantity must be increased with the increasing chroma component, so that the reproduced color signal is unnatural.
However, this conventional method involves deciding the optimum black colorant quantity satisfying the coverage restriction from the color signal in the object color space where the black colorant quantity is not given, but is not a technique in which the black colorant quantity is given in advance as the input. When the black colorant quantity is given in advance, it is desired that the black colorant quantity satisfying the coverage restriction and other color signal are decided to preserve the given black colorant quantity if possible.
A simple color processing method in which the black colorant quantity is given involves calculating the four-color color signal containing the black colorant, using the given black colorant quantity without adjustment from the color signal in the object color space, and simply preserving the ratio for four-color color signal within the coverage restriction. In this case, however, because all the four colors containing the black colorant are reduced within the coverage restriction, the black colorant quantity is not equivalent to the given black colorant quantity. Also, there is a large color difference from the given color signal, causing a problem that a color shade occurs in a dark part where the total quantity of coverage is greatly employed.
Therefore, with the conventional method, in converting the four-color color signal in the color space containing the black colorant into the color signal in the color space containing other black colorant, it was difficult to generate the color signal satisfying the coverage restriction, when realizing the colorimetric coincidence where black colorant quantities are equivalent if possible.